Extracts rewriteCSSURLs private function into middleware-ish bits.

Additionally, the middleware is only used in the combine middleware if a basePath config is provided.
Make cssUrls rewriter accessible as middleware.
Use URIjs's absoluteTo() method to resolve matched path.
Restore builtin path methods to CSS url() relative path resolution.
Add tests for cssUrls middleware that include Unicode characters.

This includes a note that our parser does not currently support the termination of a CSS hex escape with a space (as is necessary when using the shorthand syntax and the next character is a valid hex digit).
CSS url() function regex should always be preceded by a colon (as in a CSS declaration).

The rules around CSS declarations and allowed whitespace are crazy.
Add opt-in ability to rewrite import statements to cssUrls middleware.
Merge pull request #16 from zillow/cssUrls-middleware

CSS url() rewriting middleware